Propagation delay of flip flops used for counter design largely affects the speed of operation of

Delay Problem:

• In asynchronous counters, the output of the previous stage serves as the clock of the next stage.
• As the number of stages increases the propagation delay of each flip flop stage adds up resulting in the propagation delay to become significant.

The remedy of Propagation delay:

• To eliminate the propagation delay encountered in different stages, all the flip flops are provided with a common clock (Synchronous Counters).
• Thus, the output of each stage does not depend on the clock from the previous stage but only on the common clock signal and propagation delay does not add.

Synchronous UP-DOWN counter:

Johnson Counter:

Asynchronous Counter:

Asynchronous counters are those counters where the clock of the next stage is obtained from the output of the previous state.